1. Field of the Invention
The invention generally relates to electrical transformers as used in power systems for the generation, transmission and distribution of electrical energy and, more particularly, to a circuit arrangement for compensation of a DC component in a transformer, where the transformer includes a winding arrangement that is connected via connecting lines to the power system and includes a neutral point connected to earth.
2. Description of the Related Art
In power systems for the generation, transmission and distribution of electrical energy, for a variety of reasons, a DC current that is proportionately overlaid on the alternating current can form. A direct current component of this type, hereinafter known as a DC component can be created, for example, by a power converter connected to the power system, or can be a “geomagnetically induced current” (GIC).
A GIC is caused by solar wind in that the Earth's magnetic field changes so that in a conductor loop constituting network lines and earth return conductors, a flux change occurs and the electrical voltage induced therein generates the GIC. The GIC is time and direction-dependent. However, the speed at which the GIC changes is so relatively slow that it can be regarded as a DC component in the power system.
Regardless of its cause, a DC component in a transformer is always undesirable because a magnetic unidirectional flux portion is associated with it, which becomes overlaid on the alternating flux in the core of the transformer, so that the output of the transformer core is no longer symmetrical. A displacement of the operating point of the magnetic material occurs. Depending on the design of the transformer, even a very small DC component of a few 100 mA can increase the emission of operating noise by 10 to 20 dB. The displacement of the operating point of the magnetic material can lead to a significant increase in the losses in the region of 20-30%. Locally, increased heating can arise in the transformer, so that the lifespan of the insulation of the electrical winding can be impaired. In the case of a large GIC, hotspots can form on metallic parts and lead to the degradation of the insulating liquid, which can result in the formation of decomposition gases.
Various methods and devices are known for reducing a unidirectional flux portion in the core of a transformer. For example, in EP 2 622 614 B1, it is proposed to provide the transformer core with a compensation winding and, via a semiconductor switching device, to feed thereinto a compensation current the effect of which is directed contrary to the disruptive unidirectional flux portion. The advantage herein is that this compensation winding is galvanically separated from the energy network, i.e., has no connection to the primary or secondary winding system of the transformer.
A compensation winding of this type can now be realized at a reasonable cost if it is provided during the production of the transformer. Depending on the size of the DC component to be compensated, a correspondingly dimensioned installation space is to be provided, so that the dimensions of the transformer core change.
Compared with this, a retrofitting is far more effort and barely of any practical importance. If a transformer that is already in use is to be retrofitted with a DC compensation, then the transformer must be removed from the power system and the compensation winding must be installed in the interior of the transformer tank. A retrospective installation of a compensation winding is often not at all possible. In any event, the costs of a subsequent installation are very high so that, to date, retrofitting has hardly ever been performed.
Restrictions also result from the limit load of available semiconductor components. In transformers that are used as part of a high voltage DC transmission system, an induced voltage of several 1000 V can be induced in a compensation winding. A GIC can also reach a current strength of more than 50 A, as a result of which the technical implementation of the circuit is difficult.
The arrangement of semiconductor components with a limited lifespan in the interior of a power transformer or a distribution transformer that is configured for fault-free operation over several decades is also problematic.